New 250 volt circuit, no grounding bar in main panel
Hi, I have searched for this answer but I can't find a clear answer for a rookie. I am adding a 250v/20amp circuit to my main panel for a window air conditioner. I have a newer Murray main panel that was installed by an electrician, but its an old house and all the outlets are 2 prong type. So I am looking in the main panel and I see the neutral bar, with all the white wires and one bare copper wire attached to it, but no ground bar. To install the circuit, I have a new 250v/20amp dual circuit breaker I am going to install using two slots in the main panel. I am going to run wire from a 20a/250v , 2 pole 3 wire receptacle to it. Do I hook the black wire to one of the brass screws of the receptacle, the white wire to the other brass screw (and label it with red tape) and a copper wire to the ground terminal of the receptacle, then hook the black and white wires to the 2 terminals on the circuit breaker and the copper ground wire to the neutral bar, since I don't have a ground bar in the main panel?
Thanks